No major earthquakes disturbed the Earth in the week of 24-30 July 2014.
The United States Geological Survey’s real time earthquake map (which shows tremors of all magnitudes in the US and its territories and all magnitudes elsewhere) included just two of ≥M6.0.
One ‘quake on the mid-Atlantic ridge and the second, which was the largest if the week, in south eastern Mexico.
There was the usual range of tremors of M5.0 and above, concentrated along the tectonic boundaries of the western Pacific.
This week, in addition, there was a scattering of smaller tremors along the diffuse northern margin of the collision zone between India and Eurasia, marked by tremors in China and Afghanistan.
The Week’s Largest Earthquake: M6.3, Mexico
The largest earthquake to occur this week struck in south eastern Mexico, around 420km from Mexico City, in the early hours of the morning local time on 29 July. The tremor was M6.3 and occurred at a depth of just less than 100km.
Most Mexican earthquakes are associated with the subduction of the Cocos plate beneath central America, along the country’s west coast. At first glance the relationship between depth and distance from the trench (around 300km) suggests that this is not a subduction zone tremor, because the typical angle of descent at subduction zones is much steeper.
The Mexican subduction zone, however, is characterised by an unusually shallow angle of descent (known as ‘flat-slab subduction’).
The volcanic front (a line of volcanoes which, generally speaking, occur when the descending slab is at around 100km depth) doesn’t run parallel to the coastal trench but curves across Mexico to the Caribbean coast; and a look at contour maps of the descending slab indicates that the subduction zone, at the point below the epicentre, is at roughly 100km.
Together these suggest that the tremor is, after all, a subduction event.
Channel Islands Earthquakes
This week an earthquake of M4.0 struck in the English Channel, south of the island of Jersey. It follows a tremor of M3.9 earlier in July. Though small in global terms (and felt only in the immediate area) these tremors are unusual for their location.
The epicentre is far from tectonic margins and the Channel is largely stable and don’t even merit a mention in most listings of active seismic areas.
But it is active, nonetheless, though on a small scale. There’s no detailed information on the causes of the earthquake but the geological context is of a sedimentary basin which is subsiding.
The area is further influenced by the a process known as isostatic rebound. During the last Ice Age the weight of Ice on the northern parts of the British Isles forced the areas to the south upwards in a ‘see-saw’ effect. With the melting of the ice this process is reversing (on a timescale of thousands of years) with the result that the area is sinking.
US Earthquakes: M5.3, Alaska
Following last week’s M6.0 tremor in Alaska, the state produced a second large tremor (fractionally smaller at M5.9) around 500km further south. The two are unlikely to be connected, partly because of the distance between them and partly because of the different nature of the boundary.
In contrast to the earlier event, which probably occurred as a result of crustal faulting at the transition between subduction and transform margins, this week’s tremor looks likely to have been caused by lateral movement at or close to Queen Charlotte Fault, which is a strike-slip section of the boundary between the Pacific and North American plates.
Typical earthquakes, such as this week’s in Alaska, do occur – but many exhibit some degree of unusual behaviour. This is true to an extent of the Channel tremor (unusual but explicable) and also of the Mexican earthquake, whose flat slab origins are not yet fully understood.
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